1. Field of the Invention
The present invention relates to compositions consisting essentially of an organic oil and a tackifier (which itself includes a diluent), the compositions having an increased propensity to reduce dust generation from granular materials during processing of such materials compared with organic oils used alone.
2. Description of Related Art
Roofing granules, both natural and artificially color-coated, find extremely wide use in roofing and siding materials. Important applications are in granular surfaced bituminous roll roofing and asphalt shingles. The granules, which are partially embedded in one surface of asphalt-impregnated and/or asphalt-coated fiber sheet material, form a coating to provide an inherently weather-resistant, fire-resistant, and decorative exterior surface. "Weather-resistant" as used herein includes ultraviolet (UV) radiation-resistant.
Roofing granules generally consist of crushed and screened inorganic substrate granules. The substrate granules are typically coated with a ceramic coating which may include a pigment. In carrying out these coating methods, the pigment is uniformly applied to the substrate granule surface with a soluble silicate solution. The coating is usually applied in the form of a dispersion of a soluble silicate and a clay which partially or completely react together in the presence of heat to form an insolubilized ceramic coating.
Dust generation during processing and handling of inorganic granules such as roofing granules presents an environmental (air quality) issue that must be dealt with by the manufacturer and the customer, most notably at transfer points when unloading granules from shipping containers, such as trucks and railhoppers and the like, to storage tanks. Airborne particles of substrate and coating (particles range in size from 0.5 to about 30 micrometers) are typically generated during processing and handling.
Various approaches have been taken to reduce dust generation, all having certain disadvantages:
a) dust generated from white roofing granules may be reduced by reducing the amount of borax in the ceramic coating; however, this increases titanium dioxide pigment costs; PA1 b) another alternative to reduce dust generation is to use an organic oil, such as slate oil, coated onto the granules as a post-treatment coating. Increased oil usage however may decrease adhesion of roofing granules to bituminous shingles. Oil can also increase color difference between color of the originally oiled granular material and color after a given period of time. This is an especially important consideration in the manufacture of roofing granules. PA1 c) silicone oils and other adhesion agents added to the organic oil may improve adhesion of granules to asphalt-based shingles, as taught in assignee's U.S. Pat. Nos. 5,240,760 and 5,286,544; however, the problems associated with use of oil alluded to above remain; PA1 d) magnesium chloride, applied in appropriate amounts, is touted as having the ability to reduce dust generation from inorganic granules during processing of the granules, as disclosed in assignee's U.S. Pat. No. 5,368,936; however, the alkalinity of magnesium chloride-treated granules must be closely scrutinized. PA1 a) an organic oil; PA1 b) an adhesion agent (such as a silicone oil); and PA1 c) a tackifier, the tackifier consisting essentially of an organic material having a glass transition temperature of no less than about 120.degree. C. (more preferably no less than about 150.degree. C.) and a diluent present in sufficient amount to give the tackifier a kinematic viscosity ranging from about 3,000 to about 5,000 centistokes at 100.degree. C. PA1 a) providing a base granule; and PA1 b) coating the base granule with the composition of the invention, preferably to a coating weight ranging from about 0.5 to about 10 grams, more preferably about 2.5 grams to about 4.0 grams of composition per kilogram of base granules.
Given the above, it would be advantageous and an advancement in the art if the level of dust generated during processing and handling of granular materials such as roofing granules could be controlled while avoiding the disadvantages of prior approaches to the problem.